In general, flat panel displays have been utilized in systems having small installation areas as compared with cathode ray tubes. Some of the more commonly used flat panel displays include plasma display panels, electroluminescence elements and vacuum fluorescent devices. Thin film transistor-liquid crystal displays are another type of flat panel device. These displays are well suited for small screen applications but impractical for screen sizes beyond 20".
A plasma display panel consists of an array of discrete cells. Each cell is formed with a gas such as neon (Ne), argon (Ar) or helium (He) and encapsulated in glass. The plasma is formed in the gas, which may be excited by either an AC or DC field.
A large-sized plasma display panel can be easily fabricated by thick film printing technologies. Such plasma display panels have remarkable display characteristics including self-luminescence, rapid driving speed for gas discharge and a wide viewing angle ideal for wall-hanging televisions used for high definition television (HDTV). Consequently, these plasma display panels have become very popular.
The plasma display has six operating regions along the current-voltage (I-V) curve.
1) Low Current Region
There is a seed electron due to internal energy such as a neutron in a discharge tube. The seed electron is moved to an anode when an external electric field is applied, thereby causing a low current.
The amount of current is proportional to the number of seed electrons.
2) Townsend Region
When the number of electrons in the low current region exceeds a threshold number, discharge occurs and ionization increases by geometrical progression. The ionization of the plasma occurs primarily by electron collision.
3) Subnormal Glow Region
As the electric field is increased, the velocity of the electrons will exceed the velocity of the ions because of the higher mobility of the electrons. This increases the ion-electron pairs formed by electron collision, resulting in a space charge being formed in the discharge tube. The space charge results in an internal electric field redistributed into the internal gas. In the case of the discharge tube, the electric field is concentrated in the vicinity of the cathode, resulting in an initial electric field V/d of the discharge tube, where d is a distance between electrodes. As a result, ionization is increased and dV/dI has a negative characteristic.
4) Normal Glow Region
As the ion-electron pairs increase during ionization, the space charge becomes completely formed and the terminal voltage of the discharge tube is at the minimum value. At this time, dV/dI equals zero, meaning that current increases without raising the terminal voltage. Actually, as we look at the lightening in this region, we can notice that the lightening region increases.
5) Abnormal Glow Region
The entire surface of the cathode enters a glow state in the normal glow region. To increase the current, the terminal voltage of the discharge tube should be raised. At this time, the slope dV/dI has a positive resistance, and the ionization efficiency decreases. The plasma display operates in the abnormal glow region.
6) Arc Region
Increased ionization results in proportional current flow heating of the cathode surface. This causes the cathode to become damaged by ion sputtering, thereby destroying the discharge tube.
In the plasma display panel, individual control of each discrete cell is provided through a matrix structure of anodes and cathodes, as shown in FIG. 1. A discrete cell or display element is positioned at each intersection formed by the cathode-anode array and sandwiched therebetween. In AC applications, the cathodes and anodes are covered with a dielectric material, and in DC applications the electrodes are exposed.
FIGS. 2A and 2B show a timing diagram of a refreshing circuit for driving a conventional plasma display device. Initially, scanning signals are sequentially applied to cathodes K.sub.1 through K.sub.n. For each cathode scanned, anode signals representative of the image to be displayed are applied as shown in FIG. 2A.
The disadvantage of this approach is that cathodes K.sub.1 through k.sub.n cannot be scanned simultaneously. resulting in a discharge time defined by the difference between the time required to drive a single frame and the addressing time for scanning one cathode. Thus, in wide plasma display applications having a large number of cathodes the time required to drive a single frame increases resulting in a decreased discharge time. For example, with a VGA plasma display having a size of 640.times.480 with a frame time of 1/60 second, the discharge time of a cell is about 33 .mu.s. This results in decreased display brightness and therefore has limited applications for wide view display devices such as televisions.
To improve the brightness of the plasma display panel, a discharge cell has been developed to maintain a discharge subsequent to being scanned. This is called a memory method. The waveform for driving a plasma display panel according to the memory method is shown in FIGS. 3A and 3B and FIG. 4. In this embodiment, a series of pulses is sequentially applied to each cathode through an addressing operation. The addressing operation is divided into a writing phase 1 to start the discharge operation, and a sustaining phase 2. A writing pulse is applied to the cathode currently being addressed during the writing phase and a sustaining pulse is applied to every cathode during the sustaining phase irrespective of which cathode is being addressed. The time between the writing pulse of two sequential cathodes is 6 .mu.s T1, which is enough time to implement both the writing phase and the sustaining phase
As described above, when the writing pulses are sequentially applied to the cathodes, a driving signal as shown in FIG. 3A is applied to the corresponding anodes in accordance with the image to be displayed. As a result, the selected cells discharge. After discharge an electric charge particle remains in the cells for a short period. If a low voltage is applied to those cells before the disappearance of the electric charge particle, the cells continuously discharge after the writing pulses are removed.
Conversely, when a discharge is not initiated in a cell because a driving signal is not applied to its corresponding anode, the sustaining pulse would not initiate a discharge in the cell because the sustaining pulse voltage applied to the cell is below the threshold voltage to initiate discharge. Accordingly, after completion of an addressing operation, if a cell is activated to perform a discharge-sustaining operation, the discharge-sustaining time of cell becomes 5.74 ms due to both T1=6 .mu.s and T2=2.5 .mu.s in a plasma display having 480 cathodes. That is, since the discharge-sustaining time increases, the cell experiences increased brightness.
A disadvantage of this approach is that the drive time for each cell is the same. Moreover, each cell can only be controlled in the on or off state, and therefore, the color tone and brightness cannot be varied. In other words, the brightness of the discharge cell cannot be controlled in various degrees and the various color tones cannot be displayed.
To obtain multistage brightness capability, one scan of a cathode, or field, is divided into a plurality of subfields, and then the addressing and sustaining operations are sequentially performed. A method for controlling the brightness of one cell will be described with reference to FIG. 5. In this case, sixteen levels of brightness is achieved by dividing the cell into four subfields having sustaining phases equal to T.times.8, T.times.4, T.times.2, and T, respectively. A four-bit code is used, with the most significant bit (MSB) in the first subfield, the second MSB in the second subfield, the third MSB in the third subfield, and the least significant bit (LSB) in the fourth subfield. For example, if a four-bit code corresponding to a cell is 1011, the cell discharges in subfields one, three and four and maintains the discharging operation during sustaining phases T.times.8, T.times.4 and T.times.2, respectively. The discharging period of a cell according to various four-bit codes are shown in Table 1. &lt;TABLE 1&gt; ______________________________________ D3 D2 D1 D0 Discharging Period ______________________________________ 0 0 0 0 0 0 0 0 1 T 0 0 1 0 2T 0 0 1 1 3T(T + 2T) 0 1 0 0 4T 0 1 0 1 5T(4T + T) 0 1 1 0 6T(4T + 2T) 0 1 1 1 7T(4T + 2T + T) 1 0 0 0 8T 1 0 0 1 9T(8T + T) 1 0 1 0 10T(8T + 2T) 1 0 1 1 11T(8T + 2T + T) 1 1 0 0 12T(8T + 4T) 1 1 0 1 13T(8T + 4T + T) 1 1 1 0 14T(8T + 4T + 2T) 1 1 1 1 15T(8T + 4T + 2T + T) ______________________________________
With this approach, the discharge time of a cell is the difference between the field time and the addressing time of each corresponding cathode. Therefore, when the discharge time of a cell is controlled by a plurality of subfields, a new addressing time is required to each subfield, and the addressing time becomes longer in proportion to the number of subfields . As the addressing time becomes longer, a corresponding decrease in the discharge time of the cell is experienced, thereby lowering the brightness. In particular, in the case where increasing numbers of subfields are required in order to increase the number of brightness levels, more addressing period are required, resulting in the problem of reduced brightness.